Coal strain and permeability evolution in a 2D fractured model based on computerized tomography image reconstruction

Abstract

Coal heterogeneity, especially fracture distribution, affects the mechanical response and fluid seepage in the coal reservoir under complex stress conditions, which is important for exploiting coalbed methane and preventing gas outburst. The internal pores and fractures of coal samples were scanned using an X-ray industrial CT detection system in the State Key Laboratory of Coal Resources and Safe Mining of China University of Mining and Technology (Beijing). Based on CT images, 3D models of coal samples were reconstructed using MIMICS reconstruction and digital terrain model threshold segmentation. The number, size, and fractal dimension of pores and fractures were calculated basing from the vertical section of the 3D models to evaluate the heterogeneity of coal samples, and 2D numerical models of fractured coal samples were established using the Image Import module of COMSOL Multiphysics. Coal strain and permeability evolution during the reduction of gas pressure under constant confining stress were calculated in the numerical models. Results show that the distribution of pores and fractures is dominant to coal permeability. With gas pressure decreasing, the distribution of strain and gas flow rate in the coal samples is inhomogeneous, the maximum of which is observed at the fractures. Furthermore, the shrinkage and gas flow rate of the samples reduce with decreasing gas pressure, thereby alleviating the inhomogeneous distribution of coal strain and gas flow rate. As the gas pressure is decreased, the porosity of the coal samples decreases, the initial width of the fractures increases, and the ratio of current flow to initial flow decreases. When the gas pressure is changed from 2.0 MPa to 1.5 MPa, the ratio of current permeability to initial permeability of the coal samples initially decreases and then increases. Therefore, a dominant switch of effective stress and gas desorption on the permeability and mechanical response occurs. When the gas pressure is decreased, the effect of coal matrix shrinkage due to gas desorption on the permeability of the coal samples exceeds the effect of effective stress.